1. Field of the Invention
The present invention relates to an optical wavelength meter, and more specifically to one which measures the wavelength of light to be measured by using an interferometer.
2. Background Art
FIGS. 4 and 5 are for the purpose of explaining a conventional optical wave meter. FIG. 4 is a block diagram of the conventional optical wave meter. FIG. 5 is a characteristic chart of the distance accuracy of length measuring device 6 in FIG. 4, and the vertical axis in FIG. 5 corresponds to the measurement error, while the horizontal axis in FIG. 5 corresponds to the displacement.
In FIG. 4, reference numeral 1 indicates a light source to be measured, reference numeral 2 indicates a beam splitter, reference numeral 3 indicates a fixed mirror, reference numeral 4 indicates a moving mirror, reference numeral 5 indicates a direct acting mechanism which has a moving stage 5a and a guide rail 5b, reference numeral 6 indicates a length measuring device, reference numerals 7 and 8 indicate pulleys, reference numeral 9 indicates a belt, reference numeral 10 indicates a motor, reference numeral 11 indicates a light receptor, reference numeral 12 indicates a position detector, reference numeral 13 indicates a motor controller, reference numeral 16 indicates an interference fringe counter, reference numerals 17 and 18 indicate displacement counters, reference numeral 19 indicates a calculator, and reference numeral 20 indicates a display.
Light 23 to be measured, which is output from light source 1 to be measured and whose wavelength is unknown, is split into reflected light 23a and transmitted light 23b by beam splitter 2. The reflected light 23a is further reflected by fixed mirror 3 (for example, a corner-cube prism) and transmitted through the beam splitter 2 to be incident on light receptor 11. On the other hand, transmitted light 23b is reflected by moving mirror 4 (for example, a corner-cube prism) and further reflected by the beam splitter 2 to be incident on light receptor 11.
Reflected light 23a and transmitted light 23b, both input to light receptor 11, interfere with each other; thus, electric signal S24 which corresponds to the intensity of the interference light is output from the light receptor 11 to be input to interference fringe counter 16.
When motor 10 rotates, belt 9 (for example, a rubber belt) which is stretched over pulleys 7 and 8, moves in one of the directions along the light axis of light 23 (or 23b), and moving stage 5a of direct acting mechanism 5, the stage 5a being connected to the belt 9, and the moving mirror 4 which is fixed to the stage 5a also move in one of the directions along the light axis of light 23.
Accordingly, when moving mirror 4 moves in one of the directions along the light axis of light 23, electric signal S24 from light receptor 11 becomes a signal which corresponds to the cyclic variation of light intensity due to the interference. In addition, the wavelength of the electric signal corresponds to the wavelength of light 23 to be measured.
On the other hand, when moving mirror 4 moves, length measuring device 6, which consists of a scale and a sensor, outputs pulse signal S21 to position detector 12 and displacement counter 18. Each pulse of the signal corresponds to the length decided by the resolution of displacement of the sensor. At the same time, the length measuring device 6 outputs pulse signal S22, which is delayed by 90.degree. with regard to the phase to the signal S21, to displacement counter 17.
Position detector 12 counts a predetermined number of waves of pulse signal S21 from the length measuring device 6. When the detector 12 has detected that the moving mirror 4 moved by the distance corresponding to the predetermined number of waves, the detector 12 outputs position signal S25 to motor controller 13, interference fringe counter 16, and displacement counters 17 and 18. Motor controller 13 inverts the rotational direction of motor 10 every time the controller receives the position signal S25.
When position signal S25 is input from position detector 12 to interference fringe counter 16, the counter 16 starts to count the number of waves of electric signal S24. Then, when another position signal S25 is input from position detector 12 to the interference fringe counter 16 again, the counter 16 stops the counting and outputs the result K of the count to calculator 19.
On the other hand, when position signal S25 is input from position detector 12 to displacement counter 17, the counter 17 starts to count the number of waves of pulse signal S22. Then, when another position signal S25 is input from position detector 12 to the displacement counter 17 again, the counter 17 stops the counting and outputs the result N1 of the count to calculator 19.
Similarly, when position signal S25 is input from position detector 12 to displacement counter 18, the counter 18 starts to count the number of waves of pulse signal S21, and when another position signal S25 is input from position detector 12 to the displacement counter 18 again, the counter 18 stops the counting and outputs the result N2 of the count to calculator 19.
Calculator 19 conducts the process according to formula (a) shown below, based on the counting results N1 and N2 from displacement counters 17 and 18 so as to calculate displacement L of the moving mirror 4. In addition, calculator 19 conducts the process according to formula (b) shown below, based on displacement L and number K of waves of interference light from interference fringe counter 16 so as to specify wavelength .lambda. of the light 23 to be measured and outputs data of the wavelength of the light to be measured to display 20. EQU L=(N1+N2).times.[resolution of length measuring device]/2 (a) EQU .lambda.=L/K (b)
Display 20 displays the data of the wavelength of the light to be measured from calculator 19.
The optical wavelength meter shown in FIG. 4 requires a length measuring device with high accuracy in order to conduct the highly accurate measurement of the wavelength of the light to be measured. However, the accuracy of presently-obtainable length measuring devices is approximately 100 nm at best. FIG. 5 shows an example of the characteristic of the accuracy of such length measuring devices.
In addition, the length measuring device has a function of outputting an origin signal which indicates the center of the scale when the sensor passes the center of the scale.
In the length measuring device described above, measurement starting and stop points in the error characteristic shift at each measurement; therefore, the accuracy of the optical wavelength meter varies at each measurement. In this case, if fluctuation of the air and the accuracy of the optical elements which compose an interferometer are neglected, the accuracy of the optical wavelength meter is similarly determined in accordance with the accuracy of the length measuring device.
On the other hand, even if the accuracy of the distance between the starting point and the stop point is raised by correcting the predetermined displacement of the moving mirror by using a frequency-stabilized laser, there may occur (i) overshoot of the moving mirror by inertia force of the motor when the motor stops by receiving the motor inverting signal, and (ii) back-shift of the moving mirror by elastic force of the belt when the motor stops. Accordingly, it is impossible to detect accurate starting and stop points for wavelength measurement; thus, it is impossible to conduct the measurement of the wavelength based on the corrected measurement starting and stop points.